This application claims the priority of German Patent Application Serial No. 299 18 221.5, filed Oct. 15, 1999, the subject matter of which is incorporated herein by reference.
The present invention relates to a linear drive, in particular to a drive for an article of furniture. The linear drive has at least one drive motor, at least one moveable driven member operatively connected to the drive motor, and a braking element located inside the housing of the drive motor and controlled by an actuating member.
In many applications having a linear drive of this type, for example, drives for adjusting various sections of a bed, the linear drive is designed so that a DC motor drives a spindle via a worm gear. A member in the form of a spindle nut is placed on the spindle for rotation and driven by a motor only in the linear direction. The driven member is connected with the corresponding component to be adjusted through special linkage means in such a way that the linear motion is transferred to the component.
A linear drive of this type cannot be prevented from moving in the reverse direction when the drive motor is turned off. The drive assembly is self-locking, however, only if the force does not exceed a certain value, since the self-locking ability of the drive assembly is mechanically limited. Consequently, the workability of the mechanical design is severely limited.
The drive motor can be a DC motor or an AC motor. If a DC motor is employed, then the motor terminals are commonly short-circuited when the motor is not running. The resulting braking effect is referred to as a generator brake, because the DC motor operates under load as a generator and therefore has to provide a very high current under short-circuit conditions. This increases both the drive torque and the start-up limit under load. However, under frequently adverse operating conditions, so that for a desired minimum actuating speed, the mechanical self-locking effect decreases significantly due to the design of the gear stages.
Several conventional approaches employ additional braking elements so that the drive assembly remains in a rest position even under load. For example, it is known to arrange on a rotor shaft of the motor a limit stop which in conjunction with a moveable stop that can be actuated by an electromagnet, enables the motor to stop when not in operation. With this arrangement, an additional component has disadvantageously to be placed on the drive shaft, which increases the overall size of the housing of the drive motor, while the electromagnet also consumes electric energy. Moreover, a relatively large parts count is required. Only a relatively small drive power is required when articles of furniture are driven by a linear drive. Electromagnetic braking systems conventionally employed in higher power applications, for example heavy machines, where a disc brake system is used to brake the rotor shaft of the motor. The braking force is produced by a spring that takes effect when the motor is standing still. When the motor is running, the force of the spring storage device is offset by an electromagnet. However, this arrangement cannot be easily applied to actuators used with linear drives because of their much greater complexity.
It is thus an object of the present invention to provide an improved linear drive, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved linear drive of the aforedescribed type so that the braking device does not require a bigger housing, and that the braking device can be engaged without delay or at least without a noticeable delay.
These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by implementing the actuating member as an actuator, preferably as a piezo actuator, that changes its form in response to an applied electric voltage.
According to one aspect of the invention, an actuator is employed that changes its shape in response to an applied electric voltage. This type of braking device has a very simple design and can be readily implemented with any conventional drive motor. Depending on the application, the braking device can be designed so that it either releases or applies the brake when a voltage is applied to the actuator. If the linear drive is, for example, a drive for an article of furniture, then the braking device is preferably released when a voltage is applied to the actuator. A component connected to the linear drive can then be adjusted without being affected by the brake. As soon as a drive power is switched off, the braking device is activated, so that the component connected to the linear drive remains in the respective position. The drive assembly can be self-locking, so that with the braking device engaged, the component remains in that position even when load is heavier than its own weight. However, a non-self-locking drive assembly may also be employed, in which case the braking device holds the load when the motor is switched off. Since the braking elements of the braking device move only a relatively small distance from the released position into the braking position and vice versa, relatively small actuators may be employed. The necessary braking torque which is a determined by the mechanical load of the drive, is smallest on a rotor shaft of the motor. To reduce the size of the components that produce the braking torque, the actuator is coupled to a braking element that acts on the rotor shaft of the drive motor. To prevent the drive assembly from coasting after the drive motor is switched off, the actuator is connected to the electric terminals of the drive motor. The actuator is thereby activated/deactivated without delay at the same time the drive motor is switched on or off.
According to another aspect of the invention, each actuator is fixedly connected with the braking element of the braking device. The actuator advantageously releases the braking element when an electric voltage is applied. This can be achieved, for example, by changing the shape of the actuator from a circular shape to an oval shape when a voltage is applied. According to yet another aspect of the invention, two opposing braking elements can operate on the rotor shaft of the motor. With this arrangement, the braking force does not transmit a bending torque to the rotor shaft.
According to a first embodiment, an actuator is operatively associated with each braking element. Alternatively, two or more braking elements can be actuated by a single actuator by connecting between the actuator and each braking element a spring-loaded linkage or a lever. In this case, the braking force is generated by the spring(s) when the drive motor is switched off. When the drive motor is switched on, the actuator acts in a direction opposing the spring force. This is particularly advantageous if the linear drive is activated infrequently and only for a relatively short time, as may be the case, for example, with drives used with articles of furniture. Advantageously, the actuator can be integrated with the drive circuit which eliminates the need for additional wiring to supply power to the actuator. This can be accomplished easily by connecting the actuator in parallel with the drive motor. Alternatively, a relay associated with a drive motor controller can be used to control the actuator circuit. Advantageously, two pairs of back-to-back connected diodes and a switch are arranged in this circuit.